This invention relates generally to hydrostatic transmissions ("HST") commonly used with riding lawn mowers and similar small tractors. Such tractors generally use an engine having a vertical output shaft which is connected to a transaxle via a conventional belt and pulley system. Other designs use horizontal output shafts or direct shaft drive to the transaxle. The HST may be connected to an axle driving apparatus or it may be integrally formed therewith in an integrated hydrostatic transaxle ("IHT"). The general structure and benefits of HSTs and IHTs are discussed in U.S. Pat. No. 5,201,692, to Johnson and Hauser issued Apr. 13, 1993, the text of which is herein incorporated by reference.
A standard HST for a transaxle includes a hydraulic pump which is driven by the engine output shaft, and a hydraulic motor, both of which are preferably mounted on a center section containing porting to hydraulically connect the pump and motor. Rotation of the pump by an input shaft creates an axial motion of the pump pistons through use of the swash plate. The oil pressure created by this axial motion is channelled via porting to the hydraulic motor, where it is received by the motor pistons, and the axial motion of these pistons against a thrust bearing causes the motor to rotate. The hydraulic motor in turn has an output shaft which drives the vehicle axles through differential gearing.
As described, the hydraulic system has two pressure zones, the high pressure side which includes that portion of the circuit handling the movement of the fluid from the pump to the motor, and the low pressure side which includes the remainder of the circuit wherein fluid from the motor is returned to the pump. When the tractor is in reverse, the high and low pressure sides of the system are switched. It is generally understood in such designs that the pump requires more oil than is returned from the motor due to leakage from the hydraulic system into the sump. This requirement of oil is satisfied by using check valves on each side of the hydraulic system. The check valve consists of a means for preventing flow out of the system when under high pressure and a means for allowing flow into the system when under low pressure. Such check valves can be inserted directly into the center section or can be mounted in a separate check valve plate which is secured to the center section.
Furthermore, in the prior art, it is known to separately provide a mechanism for the relief of excess oil pressure (such as when neutral is desired) from the pressure side of the system. A first method of accomplishing this is by providing bleed orifices in the system from which oil will leak. However, these bleed orifices do not have the ability to close and it is seen that efficiency is lost as a result. A second method of accomplishing this is to provide a spring biased neutral valve that allows oil to pass, at a substantially constant rate, from the pressure side until a set pressure is reached, which overcomes the bias of the spring, whereby the valve will thereafter close.
While these valves work well for their intended purpose, it is seen that, among other things, these valve suffer the disadvantages of not providing smooth transition between closed and open positions and of having a rapid rate of closure whereby the neutral band is narrowed. Therefore, a need exists for an improved neutral valve.
As a result of this existing need, it is an object of the present invention to provide a combination neutral and check valve assembly. It is a further object to provide a neutral valve which has an increased neutral band. It is yet another object of the present invention to provide a neutral valve which incorporates a smooth transition between open and closed positions.